High frequency excitation system

ABSTRACT

A power module is adapted to be connected to a voltage source and to supply power to a load. The power module includes a power transistor; and a gate controller for driving the power transistor. The gate controller includes a gate transformer, and an impulse generator that extends a negative drive phase of a gate voltage to the power transistor relative to a positive drive phase of the gate voltage to the power transistor.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.11/610,091, filed Dec. 13, 2006, which is a continuation of U.S.application Ser. No. 10/868,535, filed Jun. 15, 2004, now U.S. Pat. No.7,161,818, which issued on Jan. 9, 2007, which is a continuation of PCTapplication no. PCT/EP02/014217, filed Dec. 13, 2002, claiming priorityfrom German application number 101 61 743, filed on Dec. 15, 2001. Theentire contents of these applications are herein incorporated byreference.

TECHNICAL FIELD

This invention relates to power supply systems and more particularly toa high frequency excitation system.

BACKGROUND

High frequency excitation arrangements are necessary, for example, forexcitation of a plasma for use in a gas laser (e.g., a CO₂ laser). Otherapplications include deposition of thin layers and plasma etching. Forplasma excitation, it is generally possible to input the power directlywith a DC voltage or a low-frequency AC voltage. Alternatively, thecoupling can take place capacitively through a dielectric with ahigh-frequency voltage.

German patent serial no. DE 43 22 608 C2 discloses a device thatincludes electrodes for capacitively coupling power into the plasma. Theelectrodes are connected to a voltage source through at least oneresonant circuit and at least one output stage. The output stageincludes two switching elements that can be inversely driven, and theswitching signals of the switching elements can be supplied to theresonant circuit. The resonant circuit is a series resonant circuit andthe modulation of the power coupling occurs starting from the resonantfrequency by a variation of the switching frequency. A sine form voltageis generated by the series resonant circuit, which is present at theelectrodes. The switching elements are driven by a driver circuit thatrequires a complex potential isolation of an auxiliary supply. IfMOSFETs are used as the switching elements, high power loss occurs,because the gate source capacitance of the switching elements must bereloaded through the internal resistance of the driver circuit.

SUMMARY

In one general aspect, a power module is adapted to be connected to avoltage source and to supply power to a load. The power module includesa switching bridge that includes a first power transistor and a secondpower transistor, a first gate controller for driving the first powertransistor and a second gate controller for driving the second powertransistor. The first gate controller includes a first gate transformer,and a leakage inductance of the first gate transformer forms a resonantcircuit with an input capacitance of the first power transistor. Thesecond gate controller includes a second gate transformer, and a leakageinductance of the second gate transformer forms a resonant circuit withan input capacitance of the second power transistor.

In another general aspect, a high frequency excitation system includes aload, a power module adapted to be connected to a voltage source andadapted to supply power to the load, and a power transformer connectedto the power module and connected to the load, where the powertransformer is adapted to form a series resonant circuit with the loadwhen the power module supplies power to the load. The power moduleincludes a switching bridge that includes a first power transistor and asecond power transistor, a first gate controller for driving the firstpower transistor and a second gate controller for driving the secondpower transistor. The first gate controller includes a first gatetransformer, and a leakage inductance of the first gate transformerforms a resonant circuit with an input capacitance of the first powertransistor. The second gate controller includes a second gatetransformer, and a leakage inductance of the second gate transformerforms a resonant circuit with an input capacitance of the second powertransistor.

One or more of the following features may be included. For example, theswitching bridge can further include a third power transistor and afourth power transistor, a third gate controller for driving the thirdpower transistor, and a fourth gate controller for driving the fourthpower transistor. The third gate controller includes a third gatetransformer, and a leakage inductance of the third gate transformerforms a resonant circuit with an input capacitance of the third powertransistor. The fourth gate controller includes a fourth gatetransformer, and a leakage inductance of the fourth gate transformerforms a resonant circuit with an input capacitance of the fourth powertransistor.

At least one of the first gate transformer and the second gatetransformer can include a low frequency ferrite. The first gatecontroller can further include a first impulse generator for creatingpulses to the first power transistor, and wherein the second gatecontroller further includes a second impulse generator for creatingpulses to the second power transistor. The first impulse generator caninclude a first capacitor in parallel with an first input capacitance ofthe first power transistor and a first switching element, and the secondimpulse generator can include a second capacitor in parallel with ansecond input capacitance of the second power transistor and a secondswitching element.

The load can be a gas in which a plasma can be created when the powermodule supplies power to the load. The load can be a laser-active gasmedium of a gas laser. The power transformer can include a tunableair-core coil. The power transformer can include an autotransformer. Thepower transformer can include rigid wire windings. The system canfurther include a voltage detector for detecting a voltage at the powertransformer. The system can further include a strip line for connectingthe power transformer to the power module.

In another general aspect a method of supplying power from the powermodule to ignite a plasma in a load includes, before the load includesan ignited plasma, providing power to the load at a frequency that isnon-resonant with a characteristic frequency of the load, and when theload includes an ignited plasma, providing power to the load at afrequency that is resonant with the characteristic frequency of theload.

The method can include one or more of the following features. Forexample, the method can further include monitoring a voltage supplied tothe load to determine when a plasma in the load is ignited. The methodcan further include supplying power to the load in a chain of shortpulse-width pulses to ignite the plasma.

In another general aspect a method of supplying power from the powermodule to a load can include controlling an input power to the powermodule by controlling a frequency of a signal supplied to the powermodule and controlling a load power supplied by the power module bycontrolling a duty cycle of pulses output from the power module.

The power transformer can have several functions. On the one hand, itcan convert the voltage transformation of the trapezoid wave voltage orsquare wave voltage delivered by the power module in the frequency range2 to 4 MHz into a sine-wave voltage in the range of 3 to 6 kV. Thus, thetransformer acts as sine filter or Fourier filter. The matching of theload can occur by choosing the frequency range. Therefore, a matchingnetwork, which is used often in the state of the art, is not necessary,and a significant saving of devices can be achieved. Furthermore, theload can be actively included in the power transfer, in that, forexample, the capacitance of the electrodes of the load is used as thecapacitance of the resonant circuit, so that a resonant circuitcapacitor is not necessary. The arrangement may generally be employedwith plasma methods (e.g., as used in gas lasers). However, it is alsopossible to use the arrangement for induction heating, for lightgeneration, and in inductively coupled plasma (“ICP”) applications.

A symmetrical arrangement can be provided for the power transformer inwhich the power transformer doubles the electrode voltage that can begenerated compared to an unsymmetrical arrangement having the samemaximum voltage to ground.

If the power transformer is embodied as a tunable aircore coil, anadditional ferrite core can be provided to tune the inductance and thecoupling of the power transformer. A potential separation of the primaryand secondary windings is possible if an aircore coil is used. The powertransformer can be an autotransformer having an aircore coil or with anadditional ferrite core. The windings of the power transformer can bemade of rigid wire, in which case relatively lower power losses occur atthe operating frequencies compared to when a high frequency braided wireis used.

A measuring device (e.g., a voltage detector) can be provided at thepower transformer for detecting whether ignition of a gas discharge(e.g., in the active medium of a gas laser) has taken place. Theinformation concerning whether ignition has occurred can be used forcontrolling of a turn-on sequence.

Tuning elements can be integrated into the power transformer, whichallows additional discrete components to be eliminated from thearrangement.

If the power transformer is connected to the power module by means of astrip line, a particularly good cooling of the line between the powermodule and the load may be achieved due to the large surface of thestrip line.

The switching bridge of the power module can include at least twosemiconductor switches (e.g., power transistors or power MOSFETs), whichare each driven by a gate driver. With such an arrangement, a switchingpower supply with a variable high frequency in the range of 2-4 MHz canbe realized. The supply voltage of the switching bridge can be takendirectly from the AC network by rectifying, such that the use of anetwork transformer is not necessary. A half bridge as well as a fullbridge may be used as the switching bridge. The gate driver can act asthe potential-separated driving of the semiconductor switches of theswitching bridge.

The gate driver can include at least two gate controls with a driveroutput stage and a gate transformer, respectively. In the gate driver,two clock signals, offset by 180°, can be increased to a voltage of, forexample, 12 V. Subsequent complementary emitter followers can drive thedriver output stage. The driver output stage can be a push-pull circuitof two power transistors (e.g., MOSFETs). The driver output stage can bea full bridge that includes two additional complementary emitterfollowers, respectively.

The driver output stage can generate a symmetrical square wave voltagefor excitation of the gate transformer. The advantage of using a gatetransformer is that no auxiliary supply with separated potential isnecessary, as would be the case, for example, with a conventional drivercircuit.

When the leakage inductance of the gate transformer forms a resonantcircuit with the input capacitance of the driven power transistor, asine-wave like form of the gate voltage at the driven power transistorcan develop, because the circuit is operated at a frequency close to ofits self-resonance. A sine-wave like gate current has a positive effecton the electromagnetic compatibility and can save power, depending onthe performance of the resonant circuit.

The gate transformer can be a low-frequency ferrite having a loss factorthat increases with increasing frequency and a permeability thatdecreases with increasing frequency. The resonant curve of thearrangement is broadened by both characteristics, and the usefulfrequency range of the arrangement is increased. Furthermore, the coreloss factor together with the active component of the input impedance ofthe power transistor causes a low quality of the gate resonant circuit,such that the gate resonant circuit is quickly stimulated and quicklydies out, which is advantageous for fast pulsing operation.

An impulse generator can be provided for each power transistor. Duringoperation of the switching bridge, the power transistors connected inseries can have a conducting phase of less than 180°, such that bothpower transistors are not conducting simultaneously. However, theconducting phase can be extended due to the voltage dependence of theinput capacitance of the power transistors and by the different timedelays when switching on and off, such that the pulse duty factor isreduced. This is done by the impulse generator, which extends thenegative drive phase (i.e., the off-state phase) in relation to thepositive drive phase and at the same time reduces the negative amplitudeof the gate voltage.

The impulse generator can be formed by a capacitor in parallel with theinput capacitance of the power transistor and a switching element. Inthis manner, the capacitor is only switched on during the negative drivephase in parallel with the input capacitance of the power transistor. Anauxiliary transistor can be the switching element, which is driven bythe voltage of the switched capacitor itself, but no additional voltagesupply is necessary on the secondary side of the gate transformer. Thus,in a simple way, the impulse is generated, and the pulse duty factor ofthe gate voltage are adjusted.

If the switching bridge is operated at the maximum supply voltage, theload (e.g., a gas laser with an unignited gas discharge) may absorbinsufficient power, such that the switching bridge is be damaged due toexcess voltage, depending on the construction of the switching bridge.To avoid this, precautionary measures are useful. Therefore, theignition process can be controlled to occur at such a high frequencythat only little power is coupled into the switching bridge. Inparticular, the ignition process can take place at the highest operatingfrequency of the device. If the frequency is changed after the ignitionprocess to be closer to the self-resonant frequency, an optimum powercoupling into the load can be achieved.

The voltage of the load can be monitored. For example, an ignition ofthe plasma may can be monitored by a voltage detector, which iscapacitively coupled (e.g., close to the electrodes of the load).Advantageously, a voltage sensor (e.g., a capacitive electrode) can bedisposed at the power transformer. The plasma of the load can be ignitedby an ignition sequence, in particular, by pulse chains having shortpulse-widths of the individual pulses. In this manner, the power to beswitched is limited, and damage of the power transistors of theswitching bridge may be avoided. This state is maintained until theignition of the gas discharge is detected by the voltage detector. Bothmeasures mentioned above may also be provided together.

An autonomous adjustment of the frequencies for the previously mentionedignition actions as well as the frequency limits for the power controlcan be implemented. This autonomous adjustment can take place during thestart-up of a module at the laser in a software-controlled calibrationsequence.

The input power of each power module can be controlled independently bythe frequency, and the load power for all power modules can becontrolled synchronously through the duty cycle of pulses of the signals“HF on/HF off.” The input power of each power module is thereforecontrolled independently from the load power. The duty cycle of pulsesof the signals “HF on/HF off” may occur in a duty cycle of pulsesbetween 0 and 100%, and pulse frequencies in the range of 10 Hz to 100kHz can be provided. The pulse signal is given by the central lasercontrol.

The details of one or more embodiments of the invention are set forth inthe accompanying drawings and the description below. Other features,objects, and advantages of the invention will be apparent from thedescription and drawings, and from the claims.

DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic diagram of a high frequency excitation arrangementfor operating a gas laser.

FIG. 2 is a schematic diagram of a full bridge of the power module ofthe high frequency excitation arrangement according to FIG. 1.

FIG. 3 a is a schematic diagram of a gate control directly coupled to aMOSFET to be driven, with a current supply unit, which is held at thesource potential of the MOSFET to be driven.

FIG. 3 b is a schematic diagram of a gate control coupled to a MOSFET tobe driven by means of a transformer, in which the gate control is heldat ground potential.

FIG. 4 is a schematic diagram of a gate driver of the high frequencyexcitation arrangement of FIG. 1.

FIG. 5 is a schematic diagram the temporal behavior of the gate voltageUG present at the power transistors of the full bridge of the highfrequency excitation arrangement.

FIG. 6 is a longitudinal sectional view through a power transformer withseparated windings.

Like reference symbols in the various drawings indicate like elements.

DETAILED DESCRIPTION

A power module can be connected to a load through a power transformer,in which, during operation, the power transformer creates a seriesresonant circuit with the load. Thus, the leakage reactances of thepower transformer may be used for matching the power.

As shown in FIG. 1 a high frequency excitation arrangement 1 can be usedto generate a plasma (e.g., in a laser 2). A power module 3 of thearrangement 1 includes a control card 4 that has a micro-controller 5.To supply voltage to the control card 4 or to devices arranged on thecontrol card, the alternating current (AC) supply voltage can betransformed into the voltage required by the control card 4 and itsdevice by a first auxiliary power unit 6 and a second auxiliary powerunit 7 of the power module 3. The micro-controller 5 of the control card4 is connected to a central control unit 9 through a Controller AreaNetwork (CAN) bus 8. A micro-controller on a bus master card 10 of thecentral control unit 9 converts the data of the CAN bus 8 into data of afield bus 11 (e.g., a Profi bus), which provides a connection to a lasercontroller. For service operation, the control card 4 may furthermore beconnected to a computer unit 12 through a serial interface. The actualpower generation occurs through a series resonant transformer (asexplained in further detail below) that encompasses a full bridgecircuit.

A switching bridge 14, which can be a full bridge, includes powertransistors (e.g., power MOSFETs 15-18), the gates of which are drivenby a gate driver 19. The gate driver 19 is controlled by the controlcard 4. The switching bridge 14 is supplied with a voltage directly bythe AC network through rectification, where the rectification isperformed outside the high frequency excitation arrangement 1. Thus, anintermediate DC voltage of approximately +/−300 V is present at theswitching bridge 14. The switching bridge 14 operates according to theswitching principle of class D amplifiers and, if a signal “HF on” ispresent at the bridge, feeds the intermediate DC voltage to the outputof the switching bridge 14 as a trapezoid-wave voltage in the frequencyrange 2-4 MHz. The DC input power that is measured by a measuring board20 is controlled through the frequency. The output power of the laser 2is not adjusted by this control if the DC input power but throughpulsing of the “HF on/HF off” signal with variable duty cycle controland a pulsing frequency of 10 Hz-100 kHz. The pulsing is determined bythe central control 9 and is transmitted to the control card 4.

The amplitude of the high-frequency trapezoid-wave output voltage of theswitching bridge 14 is, depending on the application, transformedthrough a power transformer 21 to approximately 4 kV and applied as asine wave voltage to the capacitive electrodes 22-25 at the dischargetubes 26-29 of the laser 2, which can be a single gas laser. The powertransformer 21 can be a single-core transformer, in which the primarycoil 30 is formed by tapping of the secondary coil 31. A capacitiveelectrode 13 at the secondary winding of the power transformer gathersthe secondary winding voltage and thus also the voltage at the load(e.g., discharge tubes 26-29). The electrode 13 is connected through alead to the measurement board 20. The detection of plasma ignition isobtained from the voltage signal at the measurement board 20, whereignition detection releases the power after the ignition of the gasdischarge. Several power modules 3 may be provided, each typicallysupplying four discharge tubes 26-29. It has been found that, if severaldischarge tubes belong to a single gas laser and are optically coupledand are connected to a common gas circulation system for exchanging gasbetween the discharge tubes 26-29 (e.g., for cooling purposes), theplasma ignition occurs for all discharge tubes 26-29 approximatelysimultaneously, i.e., as soon as one discharge tube has ignited. Thenumber of power modules used for a single laser is determined by thepower class of the laser.

As shown in FIG. 2, the switching bridge 14 of the power module 3includes power MOSFETs 15-18. MOSFETs 15 and 16 are connected with theirdrains D15 and D16 to a positive intermediate circuit DC voltage, andthe sources S17 and S18 of the MOSFETs 17 and 18 are connected to thenegative intermediate circuit DC voltage, respectively. The source S15is connected to the drain D18, and the source S16 is connected to thedrain D17. The terminals S15, D18, S16, and D17 lies at the outputvoltage U_(out) across the primary coil 30 of the power transformer 21.Thus, U_(out) is the reference potential for driving the powertransistors 15 and 16, and the negative intermediate circuit DC voltageis the reference potential for driving the power transistors 17 and 18.The secondary coil 31 of the power transformer 21 forms a seriesresonant circuit together with the plasma resistance 32 and thecapacitance 33, which is, for example, formed by the electrodecapacitance of the laser 2.

The MOSFETs 15-18 are driven by a gate control 19 a, 19 b, 19 c, and 19d (and described in greater detail below). A control 34 sets a frequencyfor an oscillator 35, and the oscillator signal is separated in adistributor 36 into clock signals Takt₁ and Takt₂, which are offset by180°. The clock signal Takt₁ is supplied to the gate controls 19 a and19 c of the power transistors 15 and 17, respectively, and the clocksignal Takt₂ is supplied to the gate controls 19 b and 19 d of the powertransistors 16 and 18, respectively.

As shown in FIG. 3 a one or more of the gate controls (collectivelydenoted as 19 x), which are directly connected to a MOSFET to be drivenand an associated current supply unit 37 can be disposed with a floatingground. The current supply unit 37 must ensure a safe separation of themains potential at a frequency of up to 4 MHz.

As shown in FIG. 3 b, one or more gate controls (collectively denoted as19 y) can be coupled potential-free to a MOSFET to be driven by atransformer with separated windings, which allows the gate driver 19 yto be held at ground potential.

Both arrangements according to FIG. 3 a or FIG. 3 b are suitable fordriving the MOSFETs 15 and 16 on the one hand and 17 and 18 on the otherhand in FIG. 2, which are at different potentials.

As shown in FIG. 4, the gate control 19 a of the power transistor 15 canmake use of the transformer coupling shown in FIG. 3 b (see also thedescription below of the gate transformer 48). The clock signals Takt₁and Takt₂ are offset by 180° and are transformed to 12V in driver ICs 40and 41 with a delay of only about 20 ns. Subsequent complementaryemitter followers 42 and 43 drive the driver output stage 44 of the gatecontrol 19 a. Each gate control 19 a, 19 b, 19 c, and 19 d includes adriver output stage 44, which are all driven in parallel by theircomplementary emitter followers 42 and 43. The driver output stage 44 ispart of a multi-stage impulse amplifier unit 45 on the low-voltage side.The driver output stage 44 can be realized as a push-pull circuit withtwo transistors 46 and 47 (e.g., MOSFETs). The driver output stage 44generates a symmetrical square-wave voltage at the gate transformer 48.The leakage inductance of the gate transformer 48 forms a resonantcircuit together with the input capacitance of the driven powertransistor 15. Thus, at the driven power transistor 15 a sine wave-likewaveform of the gate voltage develops, because the circuit is operatedwith a frequency close to its self-resonance. For the gate transformer48, a core 48 a can be made of low-frequency ferrite, which has, in theoperating frequency range of 2-4 MHz, a power loss factor that increaseswith increasing frequency and a permeability that strongly decreaseswith increasing frequency. Thus, the width of the resonant curve isincreased.

The capacitor 49 is in parallel with the input capacitance of the powertransistor 15, however, only during the negative drive phase of the gatevoltage. The switching element 50 can be a logic level auxiliary MOSFETthat is switched by the voltage of the switched capacitor 49 itself.Thus, the recovery behavior of a body diode 51 of the switching element50 can be made use of. The capacitor 49 and the switching element 50form an impulse generator.

As shown in FIG. 5, the negative drive phase 52 of the gate voltage UGis extended compared to the positive drive phase 53 because of thecapacitor 49. At the same time, the amplitude of the negative drivevoltage is reduced. The capacitor 49 therefore forms animpulse-generator circuit that reduces the drive duty cycle. This isuseful, because the conducting phase can be extended due to the voltagedependence of the input capacitance of the MOSFET 15 and due todifferent turn-on and turn-off delays.

As shown in FIG. 6, a power transformer 21 includes two concentric coilbodies 55 and 56 made of dielectric material. Coil body 55 carries thesecondary winding 57, and coil body 56 carries the primary winding 59.Each winding 57 and 59 is made of silver-coated copper wire, and theprimary winding 59 is disposed outside the secondary winding 57. Thecoil body 56 is held through holders 60, 61 at the coil body 55. Thistransformer is provided in a different embodiment instead of thetransformer of FIG. 1.

In a high frequency excitation arrangement 1 including a switchingbridge 14 and at least one power module 3 to be connected to a voltagesource and to a load, the power module 3 is connected to the loadthrough a power transformer 21 in such a manner that during operationthe power transformer 21 achieves a filtering and the power transformer21 forms a series resonant circuit with the load, and the leakagereactances of the power transformer 21 are used for the power tuning.The power transformer thus takes over several functions, such thatseparate devices can be saved.

A number of implementation have been described. Nevertheless, it will beunderstood that various modifications may be made. Accordingly, otherimplementations are within the scope of the following claims.

1. A power module adapted to be connected to a voltage source and tosupply power to a load, the power module comprising: a power transistor;and a gate controller for driving the power transistor; wherein the gatecontroller comprises: a gate transformer, and an impulse generator thatextends a negative drive phase of a gate voltage to the power transistorrelative to a positive drive phase of the gate voltage to the powertransistor.
 2. The power module of claim 1, wherein the impulsegenerator comprises: a capacitor in parallel with an input capacitanceof the power transistor, and a switching element.
 3. The power module ofclaim 1, wherein the gate transformer includes a low frequency ferrite.4. A method of driving a power module, the method comprising: connectingthe power module to a voltage source; supplying power from the powermodule to a load; controlling a gate of at least one power transistor ofthe power module with a gate controller that includes a gate transformerand an impulse generator, the gate transformer providing an input signalhaving a duty cycle to the impulse generator; and creating pulses withthe impulse generator to drive the gate of the at least one powertransistor with a reduced duty cycle relative to the duty of the inputsignal.
 5. The method of claim 4, further comprising generating asymmetrical square-wave voltage that is applied to the gate transformer.6. The method of claim 4, wherein creating the pulses includes using aswitching element and a capacitor.
 7. A method of driving a powermodule, the method comprising: connecting the power module to a voltagesource; supplying power from the power module to a load; controlling agate of at least one power transistor of the power module with a gatecontroller that includes a gate transformer and an impulse generator;creating pulses with the impulse generator to drive the gate of the atleast one power transistor with a gate voltage; and extending a negativedrive phase of the gate voltage relative to a positive drive phase. 8.The method of claim 7, further comprising reducing the amplitude of thenegative drive voltage.
 9. The method of claim 7, further comprisinggenerating a symmetrical square-wave voltage that is applied to the gatetransformer.
 10. The method of claim 9, further comprising generatingthe symmetrical square-wave voltage with a frequency that is close to aself resonance of a resonant circuit formed by an input capacitance ofthe at least one power transistor and a leakage inductance of the gatetransformer.
 11. The method of claim 7, wherein creating the pulsesincludes using a switching element and a capacitor.
 12. The method ofclaim 11, wherein creating the pulses includes using a body diode of theswitching element.
 13. A method of driving a power module, the methodcomprising: connecting the power module to a voltage source; supplyingpower from the power module to a load; controlling a gate of at leastone power transistor of the power module with a gate controller thatincludes a gate transformer and an impulse generator; and creatingpulses with the impulse generator to drive the gate of the at least onepower transistor with a reduced duty cycle, wherein creating the pulsesincludes using a switching element and a capacitor.